1. Field of the Invention
The present invention relates to an apparatus and method for aligning color channels on the basis of depth information on an image taken by an imaging device equipped with a multiple color-filter aperture (MCA), and more particularly, to an apparatus and method capable of generating a multi-focus image by correcting focuses of color channels of an image obtained by a plurality of color filters.
2. Discussion of Related Art
Image registration is a fundamental problem in image processing and computer vision fields. Among various application fields, obtainment of high-quality color images requires accurate and non-rigid registration between color channels. More specifically, color shapes can be obtained by a digital method using one of spectral sampling, spatial sampling, and temporal sampling.
In a spectral sampling system, incident light is split according to wavelength using section-specific prisms and three image sensors, and three color channels generate a perfect color image through mechanical or electronic alignment. A spatial sampling system utilizes a color filter array and a single two-dimensional image sensor, and color information lost by spatial sampling is restored by an interpolation technique referred to as demosaicing.
Meanwhile, a temporal sampling technique is used in a high-resolution device for, for example, sequential endoscopic images. Since respective image frames are obtained under different color illuminations, only one color channel is sampled from each frame. The entire color image sequence is restored by temporal interpolation requiring non-rigid alignment between color channels. In addition to temporal sampling of a high-resolution color image, an image signal processing (ISP) system of a current digital camera and a next-generation video coding system require a real-time rigid image registration technique.
A simple and intuitive image registration technique is performed by minimizing the sum of squared difference (SSD) between color channels. However, SSD-based registration has a high chance of failure because the same pixel has different pixel values in respective color channels. Recent research assumes that there is a constant relationship between pixel values in a color space, and such an assumption has become a theoretical basis of a mutual information-based registration technique.
Lately, a non-rigid registration technique using a joint histogram between color channels has been proposed for a temporally sampled medical image sequence. Such a method is only used in a main part to reduce the load of calculation, but is not appropriate to be applied to a digital camera or video coding system.